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Gary S. Fuis

Publications and source records attributed to Gary S. Fuis.

At least 37 records · Page 2Linked to original sources
USGS2012

Crustal structure of the Chugach Mountains, southern Alaska: A study of peg‐leg multiples from a low‐velocity zone

A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high‐velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg‐leg multiples generated from within a low‐velocity zone between the second and third upper crustal high‐velocity layers. The third high‐velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low‐velocity zone that produces a seismic shadow zone wherein peg‐leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high‐ and low‐velocity layers. Synthetic seismogram modeling indicates that a low‐velocity zone with transitional upper and lower boundaries generates peg‐leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long‐offset (90–225 km) data. This may be due to a lower crustal waveguide whose top is the high‐velocity (7.6 km/s) layer and whose base is the Moho. A deep (∼54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg‐leg multiples have been interpreted on some long‐range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg‐leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide‐angle reflection profiles than previously recognized.

Alaska↗
USGS2012

The crustal structure of the Wrangellia Terrane along the East Glenn Highway, eastern‐southern Alaska

Recently acquired seismic refraction data from eastern‐southern Alaska provide new information on the structure and composition of the Wrangellia and adjacent terranes. The data comprise a 160‐km‐long refraction profile along the East Glenn (Tok‐Cutoff) Highway that was collected as part of the U.S. Geological Survey's multidisciplinary Trans‐Alaska Crustal Transect program. The upper 3 km of the Wrangellia terrane and associated rocks is characterized by low compressional wave velocities (V p = 1.9, 3.3, 4.6, 5.6 km s −1 ) and high‐velocity gradients common to most onshore seismic refraction profiles. There is also clear seismic expression of the West Fork fault system as a steep, down‐to‐the‐southwest fault that separates the Peninsular terrane to the southwest and the metamorphic complex of Gulkana River to the northeast. In contrast, no seismic expression occurs for the Paxson Lake fault, which separates the Wrangellia terrane from the metamorphic complex of Gulkana River. Adjacent to the Denali fault, within the Wrangellia terrane, two high‐velocity bodies (V p = 6.6 km s −1 ) occur in the upper crust. One of these extends to ∼10‐km depth and correlates with a late Paleozoic dioritic complex, suggesting that the Wrangellia terrane is at least 10 km thick in this part of Alaska. From 5 to 23 km depth, the crust appears seismically homogeneous, with velocity increasing from V p = 6.2 to V p = 6.6 km s −1 . Beneath this level, the crust is less well resolved, although evidence exists for a low‐velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km. No identifiable mantle refraction or reflection is observed, possibly indicating a crust as thick as 55 km. The relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory‐measured velocities on pelitic schists and intermediate‐composition plutonic rocks (granites and granodiorites), both of which are recognized in Wrangellia. We interpret the seismic velocities to indicate that silicic‐to‐intermediate‐composition rocks are important constituents of the basement of this part of Wrangellia. Geologic evidence indicates that the Alaskan part of the Wrangellia terrane is a Paleozoic and Mesozoic island arc: our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of Wrangellia from Vancouver Island which was probably built on oceanic crust.

Alaska↗
USGS2012

Deep crustal structure of the Cascade Range and surrounding regions from seismic refraction and magnetotelluric data

Several regional seismic refraction and magnetotelluric (MT) profiles have been completed across the Cascade Range and surrounding geologic provinces in California, Oregon, and Washington. Analysis of three MT and two seismic refraction profiles in Oregon and a coincident MT and refraction profile in northern California show a high degree of correlation between resistivity and velocity models. The main feature that is evident in both data sets is a highly conductive (2-20 ohm m) zone that occurs at depths of 6-20 km and largely within a midcrustal velocity layer of 6.4-6.6 km/s, overlying a lower crust with velocities of 7.0-7.4 km/s. Accretionary structures in the southern Washington Cascades have been shown to be related to stress release in the area of Mount St. Helens. In order to explain the similar structures in the MT and refraction models for Oregon and California, a model is proposed involving the effects of metamorphic zonation to produce the velocity structure, combined with metamorphically produced fluids and partial melt to produce the deep conductor.

California, Oregon, Washington↗
USGS2012

A new perspective on the geometry of the San Andreas Fault in southern California and its relationship to lithospheric structure

The widely held perception that the San Andreas fault (SAF) is vertical or steeply dipping in most places in southern California may not be correct. From studies of potential‐field data, active‐source imaging, and seismicity, the dip of the SAF is significantly nonvertical in many locations. The direction of dip appears to change in a systematic way through the Transverse Ranges: moderately southwest (55°–75°) in the western bend of the SAF in the Transverse Ranges (Big Bend); vertical to steep in the Mojave Desert; and moderately northeast (37°–65°) in a region extending from San Bernardino to the Salton Sea, spanning the eastern bend of the SAF in the Transverse Ranges. The shape of the modeled SAF is crudely that of a propeller. If confirmed by further studies, the geometry of the modeled SAF would have important implications for tectonics and strong ground motions from SAF earthquakes. The SAF can be traced or projected through the crust to the north side of a well documented high‐velocity body (HVB) in the upper mantle beneath the Transverse Ranges. The north side of this HVB may be an extension of the plate boundary into the mantle, and the HVB would appear to be part of the Pacific plate.

California↗
USGS2010

Comparison of deep structure along three transects of the western North American continental margin

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver Island, and central California are now seen to be accompanied by similarities in deep crustal structure. A number of tectonic elements have been identified in the deep structure along transects in these three regions, although not all elements are present along each transect. These elements are (A) an actively subducting oceanic plate and (B) an overriding continental plate that consists of (1) a Cenozoic accretionary prism, (2) a Mesozoic accretionary prism, (3) a backstop to the Mesozoic prism, (4) a tectonically underplated body of oceanic rocks, and (5) a crustal root. The Mesozoic prism is in some cases an underthrust body (type 2a) but in other cases forms the principal component of a landward verging tectonic wedge (type 2b). The technically underplated body of oceanic rocks extends landward from the fault contact between the Cenozoic and Mesozoic prisms to a point beneath the backstop. The crustal root lies beneath the backstop and landward of the underplated body. All of these elements are interpreted to be present along the Alaskan and Vancouver Island transects. In Alaska the underplated body is interpreted to be fragments of the Kula plate; the same may be true at Vancouver Island. These two transects appear to differ in that, in Alaska, the Mesozoic prism, in one interpretation, is the principal component of a tectonic wedge (type 2b), whereas at Vancouver Island, it is an underthrust body (type 2a). Along the central California transect, active subduction is no longer taking place, and the San Andreas fault has removed the Cenozoic prism from this region of the North American plate. On the North American plate (i.e., east of the San Andreas fault), the Mesozoic prism, interpreted as the main component of a tectonic wedge (type 2b), and the backstop to the Mesozoic prism are present. There is, however, no clear evidence of tectonically underplated oceanic rocks, and the crust is thin (no root). In both Alaska and Vancouver Island, the Mesozoic prisms above the underplated bodies experienced low-pressure/high-temperature metamorphism at about the time of tectonic underplating; no such metamorphism is currently exposed in California. The metamorphism may have been caused by the underplating of young, hot oceanic crust, or, alternatively, by subduction of an oceanic ridge. The presence of a tectonic wedge (type 2b) in Alaska and California and the absence of such a wedge at Vancouver Island could arise either from the fact that in the former two locations the Mesozoic prisms were more voluminous, owing to either more rapid trench sedimentation or more rapid convergence, or to the possibility that at the latter location the Mesozoic prism was juxtaposed with the backstop primarily by strike-slip faulting.

Alaska, California↗
USGS2008

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism.

Alaska↗
USGS2002

Lower crustal deformation beneath the central Transverse Ranges, southern California: Results from the Los Angeles Region Seismic Experiment

We present a P wave velocity model derived from active source seismic data collected during the 1994 Los Angeles Region Seismic Experiment. Our model extends previously published upper crustal velocity models to mantle depths. Our model was developed by both ray tracing through a layered model and calculating travel times through a gridded model. It includes an 8-km-thick crustal root centered beneath the surface trace of the San Andreas fault, north of the highest topography in the San Gabriel Mountains. A simple mass balance calculation suggests that ∼36 km of north-south shortening across the San Andreas fault in the central Transverse Ranges could have formed this root. If north-south compression began when the “Big Bend” in the San Andreas fault formed at ∼5 Ma, 36 km of shortening implies a north-south contraction rate of ∼7.1 mm/yr across the central Transverse Ranges. If, instead, north-south compression began when the Transverse Ranges formed at 3.4–3.9 Ma, 36 km of shortening implies a contraction rate of 9.2–10.6 mm/yr. North of the San Andreas fault, the Mojave Desert crust has a low-velocity (6.3 km/s) mid and lower crust and a 28-km-deep Moho. South of the San Andreas fault, beneath the Los Angeles and San Gabriel Valley basins, there is a fast (6.6–6.8 km/s), thick (10–12 km) lower crust with a 27-km-deep Moho. Farther south still, the lower crust of the Continental Borderland is fast (6.6–6.8 km/s) and thin (5 km) with a shallow (22 km deep) Moho.

California↗
USGS2002

Report for borehole explosion data acquired in the 1999 Los Angeles Region Seismic Experiment (LARSE II), southern California: Part II, data tables and plots

The Los Angeles Region Seismic Experiment (LARSE), a joint project of the U.S. Geological Survey (USGS) and the Southern California Earthquake Center (SCEC), was conducted to produce seismic images of the subsurface in the Los Angeles region. Primary targets were major fault systems and sedimentary basins; the goal of the project was to address the earthquake hazard posed by these geologic features. The first phase of data collection (LARSE 1) was completed in 1994; the second phase (LARSE 2) was completed in 1999. A description of the 1999 survey and an overview of both phase I and II is given in Fuis and others (2001). In this report, we present the technical details for the explosion data collected in 1999.

California↗
USGS2001

Report for borehole explosion data acquired in the 1999 Los Angeles Region Seismic Experiment (LARSE II), Southern California: Part I, description of the survey

The Los Angeles Region Seismic Experiment (LARSE) is a joint project of the U.S. Geological Survey (USGS) and the Southern California Earthquake Center (SCEC). The purpose of this project is to produce seismic images of the subsurface of the Los Angeles region down to the depths at which earthquakes occur, and deeper, in order to remedy a deficit in our knowledge of the deep structure of this region. This deficit in knowledge has persisted despite over a century of oil exploration and nearly 70 years of recording earthquakes in southern California. Understanding the deep crustal structure and tectonics of southern California is important to earthquake hazard assessment. Specific imaging targets of LARSE include (a) faults, especially blind thrust faults, which cannot be reliably detected any other way; and (b) the depths and configurations of sedimentary basins. Imaging of faults is important in both earthquake hazard assessment but also in modeling earthquake occurrence. Earthquake occurrence cannot be understood unless the earthquake-producing "machinery" (tectonics) is known (Fuis and others, 2001). Imaging the depths and configurations of sedimentary basins is important because earthquake shaking at the surface is enhanced by basin depth and by the presence of sharp basin edges (Wald and Graves, 1998, Working Group on California Earthquake Probabilities, 1995; Field and others, 2001). (Sedimentary basins are large former valleys now filled with sediment eroded from nearby mountains.) Sedimentary basins in the Los Angeles region that have been investigated by LARSE include the Los Angeles, San Gabriel Valley, San Fernando Valley, and Santa Clarita Valley basins. The seismic imaging surveys of LARSE include recording of earthquakes (both local and distant earthquakes) along several corridors (or transects) through the Los Angeles region and also recording of man-made sources along these same corridors. Man-made sources have included airguns offshore and borehole explosions and vibrating-truck sources onshore. The two chief LARSE transects pass near recent moderate earthquakes, including the 1971 M 6.7 San Fernando, 1987 M 5.9 Whittier Narrows, 1991 M 5.8 Sierra Madre, and 1994 M 6.7 Northridge earthquakes. The first transect extended from San Clemente Island northeastward to the Mojave Desert (Line 1, Fig. 1), passing near the epicenter of the Whittier Narrows and Sierra Madre earthquakes. The second transect extended from west of San Clemente Island northward to the western Mojave Desert (Line 2, Figs. 1, 2), passing through the epicenter of the Northridge earthquake and near the epicenter of the San Fernando earthquake. Data along Line 1 were acquired during the years 1993-1994, and data along Line 2, during the years 1994–2000. In this open-file report and that of Murphy and others (in preparation), we present the details of the October 1999 explosion survey along Line 2, which extended from Santa Monica Bay northward to the western Mojave Desert (Figs. 1, 2). This survey is referred to as LARSE II. In this survey, 93 borehole explosions were detonated along the main north-south line and along 5 auxiliary lines in the San Fernando Valley and Santa Monica areas. These explosions were recorded by ~1400 seismographs. A variety of seismic instrumentation was used in these imaging surveys and was obtained from collaborators from around the world, including the Geological Survey of Canada (Ottawa, Canada), IRIS/PASSCAL (Socorro, NM), Lamont-Doherty Earth Observatory (Palisades, NY), Stanford University (Stanford, CA), SCEC (Los Angeles, CA), USGS (Menlo Park, CA, and Woods Hole, MA), University of Texas at El Paso (El Paso, TX), GeoForschungsZentrum (Potsdam, Germany), University of Karlsruhe (Karlsruhe, Germany), and University of Copenhagen (Copenhagen, Denmark). The reader is referred to Table 1 for instrumentation used in LARSE II.

California↗
USGS2000

Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex

We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide-angle seismic data, we find little or no sediments, low (≤6.6 km/s) P wave velocity extending down to the crust-mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ∼13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.

California↗
USGS1999

The "LARSE" Project - Working Toward a Safer Future for Los Angeles

The Los Angeles region is underlain by a network of active faults, including many that are deep and do not break the Earth's surface. These hidden faults include the previously unknown one responsible for the devastating January 1994 Northridge earthquake, the costliest quake in U.S. history. So that structures can be built or strengthened to withstand the quakes that are certain in the future, the Los Angeles Region Seismic Experiment (LARSE) is locating hidden earthquake hazards beneath the region to help scientists determine where the strongest shaking will occur.

California↗
USGS1999

El Proyecto Sismico "LARSE" - Trabajando Hacia un Futuro con Mas Seguridad para Los Angeles

La region de Los Angeles contiene una red de fallas activas, incluyendo muchas fallas por empuje que son profundas y no rompen la superficie de la tierra. Estas fallas ocultas incluyen la falla anteriormente desconocida que fue responsable por la devastacion que ocurrio durante el terremoto de Northridge en enero de 1994, el terremoto mas costoso en la historia de los Estados Unidos. El Experimento Sismico en la Region de Los Angeles (Los Angeles Region Seismic Experiment, LARSE), esta localizando los peligros ocultos de los terremotos debajo de la region de Los Angeles para mejorar la construccion de las estructuras que pueden apoyar terremotos que son inevitables en el futuro, y que ayudaran a los cientificos determinar donde occurira el sacudimento mas fuerte y poderoso.

Fact Sheet↗
USGS1996

The geology and mechanics of formation of the Fort Rock Dome, Yavapai County, Arizona

The Fort Rock Dome, a craterlike structure in northern Arizona, is the erosional product of a circular domal uplift associated with a Precambrian shear zone exposed within the crater and with Tertiary volcanism. A section of Precambrian to Quaternary rocks is described, and two Tertiary units, the Crater Pasture Formation and the Fort Rock Creek Rhyodacite, are named. A mathematical model of the doming process is developed that is consistent with the history of the Fort Rock Dome.

Professional Paper↗
USGS1994

Crustal velocity structure of the northern Yukon-Tanana upland, central Alaska: Results from TACT refraction/wide-angle reflection data

The Fairbanks North seismic refraction/ wide-angle reflection profile, collected by the U.S. Geological Survey Trans-Alaska Crustal Transect (TACT) project in 1987, crosses the complex region between the Yukon-Tanana and Ruby terranes in interior Alaska. This region is occupied by numerous small terranes elongated in a northeast-southwest direction. These seismic data reveal a crustal velocity structure that is divided into three upper-crustal and at least two middle- to lower-crustal domains. The upper-crustal domains are delineated by two steeply dipping low-velocity anomalies that are interpreted as signatures of the Victoria Creek fault, and the Beaver Creek fault or a fault buried by the Beaver Creek fault. This tripartite upper crust extends to 8-10 km depth where a subhorizontal interface undercuts the northern and central domains. Beneath the northern domain, this interface is interpreted as the southeastwardly dipping boundary between the Tozina and Ruby terranes. The continuation of this interface beneath the central domain suggests that it may represent the detachment or basal thrust for thin-skinned tectonic amalgamation of the terranes caught between the Yukon-Tanana and Ruby terranes. The lower crust and Moho reflection exhibit differences from north to south that define at least two lower-crustal domains, interpreted as the Yukon-Tanana and Ruby terranes. Finally, the crustal thickness along the profile is nearly uniform and ranges from 31 to 34 km. Our data suggest that after initial thin-skinned amalgamation of the various terranes, this region experienced thick-skinned tectonic reorganization via strike-slip faulting. This interpretation supports a model in which at least one strand of the Tintina fault exists in this important region of Alaska.

Alaska↗
USGS1988

Data report for 1980-1981 seismic-refraction profiles in the western Mojave Desert, California

During 1980 and 1981, the U.S. Geological Survey collected seismic refraction data in the western Mojave Desert (fig. 1). The seismic refraction data extends from north of the Garlock Fault, south through the Mojave Desert, and across the San Andreas Fault into the San Gabriel Mountains. This report describes and documents the recorded data, which was collected over three field sessions. In total thirteen shots were fired at 11 different shotpoints (Table 1). The experiment consisted of five deployments of a set of 100 portable seismic recorders, during which seismic energy from one or more shots was recorded. The data set was acquired with five deployments of a set of 100 portable seismic recorders.

California↗